The biggest war game in US military history, staged this month at a cost of £165m with 13,000 troops, was rigged to ensure that the Americans beat their "Middle Eastern" adversaries, according to one of the main participants.

General Paul Van Riper, a retired marine lieutenant-general, told the Army Times that the sprawling three-week millennium challenge exercises, were "almost entirely scripted to ensure a [US] win".

He protested by quitting his role as commander of enemy forces, and warning that the Pentagon might wrongly conclude that its experimental tactics were working.

When Gen Van Riper agreed to command the forces of an unnamed Middle Eastern state - which bore a strong re semblance to Iraq, but could have been Iran - he thought he would be given a free rein to probe US weaknesses. But when the game began, he was told to deploy his forces to make life easier for US forces.

"We were directed... to move air defences so that the army and marine units could successfully land," he said. "We were simply directed to turn [air defence systems] off or move them... So it was scripted to be whatever the control group wanted it to be."

The Army Times reported that, as commander of a low-tech, third-world army, Gen Van Riper appeared to have repeatedly outwitted US forces.

He sent orders with motorcycle couriers to evade sophisticated electronic eavesdropping equipment. When the US fleet sailed into the Gulf, he instructed his small boats and planes to move around in apparently aimless circles before launching a surprise attack which sank a substantial part of the US navy. The war game had to be stopped and the American ships "refloated" so that the US forces stood a chance.

"Instead of a free-play, two-sided game as the joint forces commander advertised it was going to be, it simply became a scripted exercise. They had a predetermined end, and they scripted the exercise to that end," Gen Van Riper said. He said he quit when he found out his orders were being over ruled by the military coordinators of the game.

Vice-Admiral Marty Mayer, one of the coordinators, denied claims of fixing. "I want to disabuse anybody of any notion that somehow the books were cooked," he said.

The games were designed to test experimental new tactics and doctrines advocated by the defence secretary, Donald Rumsfeld, and were referred to in Pentagon-speak as "military transformation".

The transformation is aimed at making US forces more mobile and daring, but Gen Van Riper said that the "concepts" the game were supposed to test, with names such as "effects-based operations" and "rapid, decisive operations", were little more than "slogans", which had not been properly put to the test by the exercise.